The present invention relates to an engine controller, and in particular to a method for controlling torque output of an engine during idling.
Torque based control systems are used in motor vehicles to compute a torque request of a driver of the vehicle as a function of speed of an engine of the vehicle and a position of an acceleration pedal of the vehicle. FIG. 1 illustrates a torque output map used in a typical torque based control system. The typical torque based control system controls the torque output of the engine of the vehicle on a crankshaft according to the engine torque request read from the torque output map. The torque output map includes an X-axis 10 having values for the speed of the engine (in, e.g., revolutions per minute). The torque output map also includes lines 12 representing a percent of depression of the acceleration pedal. In FIG. 1, the top line 12a represents full depression of the acceleration pedal and the bottom line 12b represents zero depression of the acceleration pedal. The torque output map further includes a Y-axis 14 having values for the desired output torque. The desired output torque can be determined from the torque output map by reading the value on the Y-axis 14 corresponding to a meeting point of the line 12 representing the percentage of depression of the acceleration pedal and the engine speed on the X-axis 10. Once the desired output torque is determined, the torque based control system sets the torque of the crankshaft equal to the desired output torque.
During idling (i.e., when the engine is at idle speed), the desired output torque should be set at zero such that the vehicle does not have a positive output torque or negative output torque on the crankshaft. Therefore, at idle speed, the line 12b for zero depression of the acceleration pedal should meet the idle speed on the X-axis 10 (i.e., the torque output map, or Y-axis value, is zero). However, the engine speed can sometimes increase or decrease during idling. For example, the vehicle may experience a change in temperature during idling. When the engine speed increases, the desired output torque read from the torque output map will decrease. Vehicles with torque based control systems can include idle speed controllers (typically a PI-controller) to counteract the increase in idle engine speed. Therefore, when the engine speed increases during idling, the idle speed controller decreases the engine speed until the line 12b for zero depression of the acceleration pedal once again meets the idle speed on the X-axis 10 to thereby set the desired output torque at zero. Likewise, when the engine speed decreases, the desired output torque read from the torque output map will increase. When the engine speed decreases during idling, the idle speed controller increases the engine speed until the line 12b for zero depression of the acceleration pedal once again meets the idle speed on the X-axis 10 to thereby set the desired output torque at zero. However, the idle speed controller can take time to counteract any change in engine speed during idling. The torque output map is normally designed such that the nominal engine idle speed is the speed where line 12b in FIG. 1 intersects the X-axis. However, during engine operation, the engine management system may use another set point speed for the idle speed controller, and this will be the actual engine idle speed. A reason for increasing the engine idle speed may be to heat the catalyst during startup.
Accordingly, a quick response to changes in engine speed during idling is desired.